1. Field of the Invention
The present invention relates to an ink jet recording method and an ink jet recording apparatus in which an ink jet head is employed to record an image on a recording medium.
2. Description of the Related Art
In an ink jet recording apparatus in which an image is recorded on a recording medium by using an ink jet recording head having a plurality of ejection orifices to eject ink, small ink droplets called satellites are often generated with a main ink droplet (hereinafter referred to also as a “main droplet”) and ejected through the ejection orifice of the recording head. FIGS. 3A to 3D show behaviors of the satellites generated. With the lapse of time in sequence of FIGS. 3A, 3B, 3C and 3D, an ejected ink droplet is separated into a main droplet 10 and a group of small plural satellites 112. Also, though not shown, when the main droplet strikes against the recording medium, very small ink droplets may generate in the form of splashed ink from the recording medium. If those small ink droplets (hereinafter referred to also as “ink mists” or simply as “mists”) attach to the ejection orifice surface of the recording head, an ink pool may generate on the ejection orifice surface and may cause an ejection failure. In particular, when the ejection duty of ink ejected per unit time is high, the ink mists generate in a larger amount and a possibility of the ejection failure increases.
Various methods have hitherto been proposed to avoid the ejection failure caused by the ink mists. According to Japanese Patent Laid-Open No. 05-309874, for example, the number of passes in multi-pass recording is increased in the case of a high duty as compared with the case of a low duty. This method is able to reduce an amount of ink ejected per unit time and to reduce an amount of ink mists generated.
Also, according to Japanese Patent Laid-Open No. 06-344574, the number of times of wiping is increased in the case of a high duty in which the ink mists are relatively easy to occur as compared with the case of a low duty in which the ink mists are relatively hard to occur. This method is effective in avoiding ejection failure because even when the ink mists are attached to the ejection orifice surface, the attached ink mists are immediately cleaned off.
With the methods disclosed in the above-cited Japanese Patent Laid-Open Nos. 05-309874 and 06-344574, however, the recording speed reduces because the number of passes and the number of times of wiping are increased in the case of a high duty.
Meanwhile, in the recent ink jet market, there has been a demand for outputting an image with high quality, like photographic quality, at a high speed. To satisfy such a demand, an ink jet head has an increasing tendency toward a smaller droplet size, a higher density of ejection orifices, and a larger length. Correspondingly, a recording apparatus equipped with such an ink jet head faces an increasing tendency toward a higher head scan speed and a higher driving frequency.
In those situations, the amount of ink mists generated in the case of a high duty and attached to the ejection orifice surface (i.e., the surface in which the ejection orifices are formed) increases more and more. For that reason, any action to avoid the ejection failure caused by the attached mists is required.
As a result of intensive study in the relationship between the ink ejection duty and the amount of ink mists attached to the ejection orifice surface, new findings have been confirmed in two points, i.e., 1) the amount of the attached ink mists generally tends to increase in the case of a high duty, but the amount of the attached ink mists is not always so increased as to cause the ejection failure even in the case of a high duty, and 2) when two adjacent ejection orifice groups each have a high duty, there is a tendency that the amount of the attached ink mists is so increased as to cause the ejection failure. Those new findings will be described in more detail below with reference to FIGS. 4, 5A and 5B.
FIG. 4 shows an ink droplet ejection state when a secondary color image is recorded by continuously ejecting ink droplets through ejection orifices in a half of each of two adjacent ejection orifice groups while moving an ink jet head (recording head) H1001, which is capable of ejecting inks in different colors, at a high speed relative to a recording medium 3. The recording head H1001 is moved in a direction indicated by an arrow in FIG. 4, and ejection orifices H1107 in the recording head H1001 are arrayed in a direction substantially perpendicular to the head moving direction. When image data has a high duty, an ejection energy generator (not shown) corresponding to each ejection orifice of each of the ejection orifice groups used for forming the secondary color is driven at a high driving frequency. Therefore, as an ink droplet 10 ejected through the ejection orifice H1107 moves toward the recording medium 3, air having viscosity and residing around the ink droplet 10 is also moved while being entrained with the movement of the ink droplet 10. As a result, the vicinity of the ejection orifice surface tends to have lower pressure than the surrounding of the recording medium 3. The ambient air is caused to flow toward a depressurized area (i.e., the vicinity of the ejection orifice surface), thereby generating air streams 1 that curl upward from the recording medium 3. Then, it has been confirmed that mists are splashed when the ink droplet 10 and satellites flying in accompanying with the ink droplet 10 strike against the recording medium 3. The splashed mists are attracted toward the ejection orifice surface side of the recording head H1001 under the action of the air streams 1.
When a high-duty image is recorded under such a condition with the so-called multi-pass recording in which the image is recorded by moving the recording head many times over the same area of the recording medium, ink mists 47 are attached to the ejection orifice surface of the recording head H1001 as shown in FIGS. 5A and 5B. FIG. 5A shows a state of mists attaching to the ejection orifice surface when a high-duty primary color image (e.g., a cyan image with a 36% duty) is formed, and FIG. 5B shows a state of mists attaching to the ejection orifice surface when a high-duty secondary color image (e.g., a mixed image with a 36% duty of a cyan image with a 18% duty and a yellow image with a 18% duty) is formed by high-duty ejection from each of the adjacent ejection orifice groups. As seen from FIGS. 5A and 5B, because ink droplets are ejected from the two ejection orifice groups at a high frequency in the case of a secondary color, a tendency to cause a pressure reduction is increased in comparison with the case of a primary color, and the ink mists 47 are more apt to reach the ejection orifice surface of the recording head H1001. Thus, even at the same high duty, the mists are attached to the ejection orifice surface in such a small amount as not causing the ejection failure in the case of a primary color, while the mists are attached to the ejection orifice surface in such a large amount as possible causing the ejection failure in the case of a secondary color in which the image is recorded by using the adjacent ejection orifice groups.
As a result of more detailed analysis conducted by the inventors, it has been confirmed that, in a head having the ejection orifice surface subjected to water repellent treatment substantially all over the ejection orifice surface, the ink mists tend to attach in a larger amount in areas farther away from the ejection orifices. For example, when the secondary color is recorded by using the adjacent ejection orifice groups, a large number of ink mist masses 48 having grown to sizes of about 300 μm to 500 μm diameters are produced in areas away from the ejection orifices by a distance of about 500 μm to 1 mm, as shown in FIG. 5B. In the water repellent area of the ejection orifice surface where those ink mist masses 48 are present, a contact angle for water (ink) is large and the ink has high fluidity. Hence, the ink mist masses 48 are easily movable on the ejection orifice surface with the movement of the recording head, and eventually reach the ejection orifices. Consequently, the ink mist masses 48 are drawn into the ejection orifices of one to several nozzles, thus resulting in non-ejection of the ink.
As described above, in the current situation in which ink droplets are ejected at a high driving frequency in a recording head having a small droplet size, a high density of ejection orifices, and a large length, while moving the head at a high speed, any action must be taken because the amount of attached mists increases beyond an allowable range when both of adjacent ejection orifice groups have high duties.